Quote Originally Posted by Q.G. View Post
Seen from under an angle, not from straight behind, a circle appears as an ellips, is smaller.
Make the hole ellips shape, and from the side the long axis points to, the effect is negated. But only from that side, and only at a certain angle/distance from the image's centre.

I don't know how that would creat astigmatisme, except that the shape is echoed in the pencil of light's shape. But that's not astigmatism.
Here is the results of off-axis resolution tests done decades ago. Notice the difference in off-axis resolution between tangential and radial target lines. Perhaps this isn't astigmatism, but the results are the same. According to Pinhole Designer with a user constant of 1.5, the pinhole for optimum on-axis resolution in this test would have been .013 inches.